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Title: De-Stabilization of Streambanks by Removal of Invasive Species in Canyon de Chelly National Monument, Arizona

Author
item Bankhead, Natasha
item Simon, Andrew
item JAEGER, KRISTIN - COLORADO STATE UNIVERSITY
item WOHL, ELLEN - COLORADO STATE UNIVERSITY

Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2008
Publication Date: 7/20/2008
Citation: Pollen-Bankhead, N., Simon, A., Jaeger, K., Wohl, E. 2008. Destabilization of Streambanks by Removal of Invasive Species in Canyon de Chelly National Monument, Arizona. Geomorphology, doi:10.1016/j.geomorph.2008.07.004.

Interpretive Summary: As part of a study to investigate the causes of channel narrowing and incision in Canyon de Chelly National Monument, the effects of Tamarisk and Russian-olive on streambank stability were evaluated. The National Park Service in Canyon de Chelly are currently investigating the effects of removal of the invasive species, Tamarisk and Russian Olive, on streambank stability. The park service hopes that removal of the invasive species will cause the channel and landscape to return to how it looked approximately a century ago, when multiple, shallow channels filled the canyon instead of one, narrow, deep channel. In this study, root strengths and densities were measured and used in a root-reinforcement model to estimate the additional strength added to the banks by the roots of the invasive species. Additional strength due to roots ranged from 0 to 6.9 kPa for tamarisk and 0 to 14.2 kPa for Russian olive. Average root-reinforcement over the entire bank profile was 2.5 and 3.2 kPa for tamarisk and Russian olive respectively. The potential effects of vegetation removal on failure extent and frequency were evaluated at two different sites using a bank stability and toe erosion model. Banks in the modeled sections are 4.3 and 3.9 m high, respectively and are dominated by sandy materials except near the toe where fine-grained deposits provide between 2.2 and 4.0 kPa of cohesion. Because the bank materials are dominated by sands, cohesion provided by roots is significant to bank stability, providing an average 2.8 kPa of cohesion to otherwise cohesionless bank materials. Bank retreat rates at one site following vegetation removal have approximately doubled (from an approximate rate of 0.7 – 0.8 m/y between 2003 and 2006 to 1.85 m/y during the year modeled. Removal of further riparian stands of Tamarisk and Russian-olive will lead to bank instabilities and accelerated widening in Canyon de Chelly National Monument, thereby increasing the likelihood of channel shape returning to a system of shallower, multiple channels rather than the single, narrow, meandering pattern that currently dominates the Canyon.

Technical Abstract: As part of a study to investigate the causes of channel narrowing and incision in Canyon de Chelly National Monument, the effects of Tamarisk and Russian-olive on streambank stability were evaluated. The National Park Service (NPS) is currently engaged in an experimental program to remove the invasive species Tamarisk (Tamarix ramosissima) and Russian-olive (Elaeagnus angustifolia) from the margins of Canyon de Chelly, Arizona. The Park Service hopes that this program will aid in returning the channels to the wide and shallow geometry characteristic of the region a century ago by making banks more susceptible to erosion. In this study, root tensile strengths and distributions in streambanks were measured and used in combination in a root-reinforcement model, RipRoot, to estimate the additional cohesion provided to layers of each streambank. The additional cohesion provided by the roots in each 0.1m layer ranged from 0 to 6.9 kPa for Tamarisk and from 0 to 14.2 kPa for Russian-olive. Average root-reinforcement values over the entire bank profile were 2.5 and 3.2 kPa for Tamarisk and Russian-olive, respectively. The implications of vegetation removal on bank stability and failure frequency were evaluated in two incised reaches by modeling bank-toe erosion and bank stability with and without vegetation. Banks in the modeled sections are 4.3 and 3.9 m high, respectively and are dominated by sandy materials except near the toe where fine-grained deposits provide between 2.2 and 4.0 kPa of cohesion. The effects of a series of 1.0 and 1.5 m-deep flows on bank-toe erosion, pore-pressure distributions and bank stability were evaluated first. In addition, bank stability model runs were carried out using iterative modeling of toe erosion and bank stability using a discretized flow and groundwater record for one year. Results showed that the effects of root reinforcement provided by Tamarisk and Russian-olive have a significant impact on bank-stability and bank-failure frequency. Because the bank materials are dominated by sands, cohesion provided by roots is significant to bank stability, providing an average 2.8 kPa of cohesion to otherwise cohesionless bank materials. Bank retreat rates at one site following vegetation removal have approximately doubled (from an approximate rate of 0.7 – 0.8 m/y between 2003 and 2006 to 1.85 m/y during the year modeled. Removal of further riparian stands of Tamarisk and Russian-olive will lead to bank instabilities and accelerated widening in Canyon de Chelly National Monument, thereby increasing the likelihood of channel morphology returning to a wide, braided system rather than the narrow, meandering pattern that currently dominates the Canyon.